Rockwell Automation T8842 Trusted Versatile FTA - Manual (Page 15)

Trusted
TM
Versatile FTA T8842
Issue 8 Aug 12 PD-T8842 15
Monitored Valve Outputs
For a Trusted TM TMR Valve Monitor Module T8449, an output configuration as shown in Figure 6 is
required to drive the safety valve. The valve position is monitored by the next channel configured as an
input. This input may be an analogue position sensor, which is wired as shown in Figure 7. This input
expects a 4-20mA current loop from the position sensor. It may also be wired from a set of limit
switches arranged in a resistor network. This network is powered from 24V and received as a voltage
input as in Figure 5. The network should be arranged so that the voltage seen at the channel input
passes through a series of steps as the switches are activated in turn. The steps should continue in
one direction through the whole valve travel, or the valve monitor module will not be able to determine
the valve position.
The diagrams below shows an example switched voltage circuit, using Open, Nearly Open and Closed
switches. Each switch is wired to close to the clockwise side of travel (note: ‘O’ will close when ‘not
open’). The resistor values shown will ensure that the lowest value resistor will override the state of the
others. The thresholds are programmed into the module System.INI and set the target states for the
valve test. They also allow room for open and short circuit detection.
Figure 9 Position Limit Switches and circuit
Figure 10 Voltage Input with Limit Switch Position
O NO
C
3
2
1
24V
0V
1K
10K
O NO C
1K2 220R 4K7
V
2.2
5.7
12.8
20.4
Threshold 4V
Threshold 10V
Threshold 18V
open
nearly open
travel
closed